1. Field of the Invention
This invention relates to temperature measuring devices, and, in particular, to temperature measuring devices having deformable mounting heads thereon.
2. Description of the Prior Art
It is often important to those in the art of power generation to accurately ascertain the temperature of the metal of the power generation apparatus or of the motive fluid utilized therein. The motive fluid, commonly steam, is confined within a casing member to convert the energy carried in the steam into useful mechanical work. The casing may itself be surrounded by one or more concentric outer casing members.
In order to ascertain the temperatures within the metal of the inner casing member or the temperature of the motive fluid contained therein, it is necessary to provide temperature measuring devices disposed at various predetermined points within the casing or within the motive fluid flow. There are, in general, two main types of devices available for obtaining the temperature at a predetermined point within the inner casing of a turbine apparatus. The casing is usually swept by a flow of high velocity fluid over both its interior and exterior surfaces. The first type of device comprises a temperature sensor, usually a thermocouple junction, encased in a metallic sheath. The sensor used in this type of device may or may not be electrically isolated from the casing. The second type of device comprises only unsheathed thermocouple junctions which are electrically in contact with the casing whose temperature is being measured.
One arrangement which exemplifies the first type of device requires the provision of aligned bores, one of which extends completely through the outer casing and the other extending a predetermined distance into the metal used to fabricate the inner casing. An elongated, hollow protective tube, having a closed end thereon, extends into the aligned bores provided in the casings and through the annular space therebetween. The thermocouple junction is inserted into the protective tube to a point adjacent to, but not in contact with, the closed end thereof. The temperature measured by the junction is monitored and recorded by suitable external instrumentation.
There are often considerable errors, however, in the temperature measurements obtained by this arrangement. Such errors are due to distortions in the temperature field created by the cavity provided for the device. In addition, layers of stagnant fluid surrounding the end of the thermocouple sheath and the end of the protective tube contribute to the measurement errors. Besides measurement errors, complications involved in the installation of the aligned bores, plus undesirable shop disruptions the process entails, makes this arrangement economically disadvantageous. A similar arrangement, used to measure the temperature of the steam flow within the inner casing, suffers from the same disadvantages.
An alternative arrangement exemplifying the first type of devices disposes a bore within the material of the inner casing, the bore terminating adjacent the point of interest. The thermocouple junction is then inserted into the bore. The junction is maintained near to, but not touching, the bottom of the bore by the gripping action of a fitting rigidly holding the sheath in place. The fitting is, in turn, threaded into a boss protruding from the surface of the casing. The measurements obtained by the thermocouple junction are often misleading, however, since the junction is not disposed in contact with the casing and since the substantially unfilled void distorts the basic temperature field.
A third example of the first type of devices, similar to the second example described above, disposes the thermocouple junction within a bore in the inner casing member, and maintains the junction in contact with the casing with a spring loaded retainer. A bulky housing, which protrudes above the exterior surface of the inner casing, houses the retainer. This scheme may only be used, of course, where there is available a sufficiently large annular space between casings in which to dispose the housing. Such a housing would, of course, interfere with the fluid flow within the annular space.
This arrangement is also disadvantageous since there is still present a substantially unfilled cavity which disrupts the temperature field. Further, vibrations may be transmitted through the spring mounting to the junction, resulting in premature failure of the temperature measuring device. The provision of the threaded connection, moreover, and the boss which must be provided to maintain the threads above the stress field of the casing, entail undesirable fabrication complications.
The second main type of devices for obtaining the temperature of a predetermined point are best exemplified by Macatician, U.S. Pat. No. 2,829,185; Kanne, U.S. Pat. No. 2,856,341; Lederer, U.S. Pat. No. 2,000,489; and States, U.S. Pat. No. 2.012,112. These patents all disclose unsheathed thermocouple junctions which are electrically in contact with the body member whose temperature is being measured.
These devices all exhibit the drawback of having the junction grounded to the body member. Such a configuration leads to the possibility of spurious electrical currents which distort the measurements obtained. Also, these devices are limited only to the use of thermocouple junctions as the temperature sensors. All these devices have a mechanically or a metallurgically undesirable connection between the body and the device, either at the mouth of the bore or in the bore itself. All of these devices exhibit a construction which is fragile and unable to withstand buffeting by the fluid flow which sweeps along the external surface of the inner casing, and, for this reason, are inapplicable to the problem at hand. Further, all devices shown in the above Patents are susceptible to unacceptable temperature measurement errors. Macatician, for example, leaves a vacant gap around the sensing junction which tends to disturb the temperature field. Kanne also leaves a substantially unfilled gap around his sensing junction. Lederer, on the other hand, surrounds his device with a material different from the material of the body whose temperature is being measured. Also, his sensing junction is unsuitable for measuring internal metal temperatures since the large surface areas exposed by his device will be adversely affected by the temperature of the fluid flow sweeping over the external surface of the casing. It is to be noted, in this regard, that States is particularly unsuited for any mechanical temperature measurement application.
It is apparent that the prior art arrangements for measurement of the temperature of the metal casing are inadequate. A temperature measuring device that provides accurate measurement of the temperature at a predetermined point within the casing or when inserted into the fluid flow itself is a prime requisite. Further, the presence of unfilled voids which distort the temperature field must be avoided. The measurement device must be in intimate contact with the body at the point at which the temperature measurement is desired in order to avoid measurement errors. The device must also be securely fastened to the innermost casing so as to be able to withstand the battering imposed thereon due to the fluid flow sweeping over the external surface of the inner casing. At the same time, however, the attachment must not unduly stress the casing. The device must also be ruggedly constructed to withstand the forces imposed thereon if immersed in the fluid flow confined within the casing. The sensor must be electrically isolated from any extraneous electrical currents which may distort the measurements obtained.